Recent studies provide evidence that microbial communities residing in the human gut play a key role in the development and modulation of the host immune response. For instance, the presence of particular Clostridium species has been shown to induce specific T-cell repertoires [Atarashi, et al. (2011) Induction of colonic regulatory T cells by indigenous Clostridium species. Science 331(6015):337-341]. Despite the complexity of the gut microbiome, the presence or absence of specific bacterial species can dramatically alter the adaptive immune environment.